Error 399287 - MFA verification fails, phone is the only method (work account, sole admin)

Dear Microsoft Support Team,

I am posting on behalf of my father, who is the owner of the affected account and the only administrator of the tenant. He cannot post himself because he is locked out of the account. He is available to provide any identity, subscription or billing verification details required.

The issue: the password step succeeds, but the identity verification step always fails with the following message:

"Sorry, we're having trouble verifying your account. Please try again."

Error Code: 399287

Request Id: a7b9cb44-c4c1-4aa2-8b28-5577cca71700

Correlation Id: 7643fe32-6fd3-4a3a-9569-432d79d926e6

Timestamp: 2026-09-02T21:43:44Z

Account information:

• Affected account: ******@lebym.onmicrosoft.com
• Tenant domain: lebym.onmicrosoft.com
• Tenant ID: d68a4d81-71dc-432c-bd9f-071f55d94e3f
• Tenant region scope: SA

What we have already tried:

• Both offered verification methods, SMS and phone call, to the same registered number. Both fail with the same error.
• Waiting several hours before retrying, in case the block was temporary. The error persists.
• Signing in from different devices, browsers and networks. Same result every time.
• Reaching the security info page (aka.ms/mfasetup) through an existing Office session on a PC where the account was still signed in. The page requires a fresh MFA challenge, which fails with the same error, so Microsoft Authenticator cannot be added as an alternative method.

Why this cannot be solved within the tenant: phone is the only authentication method registered on this account, and changing it requires signing in, which is exactly what is blocked. The account owner is the only administrator in this tenant, so there is no other global admin who could reset the MFA methods.

We have also tried contacting support by phone (spending more than 1 hour by phone), but the automated system does not route the call to a human agent, and we cannot open a ticket from the Microsoft 365 admin center because the portal itself requires the sign-in that is failing.

We would appreciate help getting this case routed to the Data Protection team, so the MFA methods on the account can be reset and Microsoft Authenticator can be registered instead of SMS.

Please let us know if any additional information is needed. Thank you for your support.